Organ transplantation is an effective therapy for end-stage organ failure, but is severely limited by a shortage of donor organs. Numerous people die each year while waiting for an organ. In addition to the difficulty in obtaining donor organs, the expense of organ transplantation also limits the number of organ transplant operations carried out.
The possibility that animals could provide an alternative source of donor organs and tissues has stimulated much debate. Xenotransplantation clearly has the potential to alleviate the suffering and mortality associated with donor organ shortages, yet certain safety concerns are associated with the procedure. Concerns include infectious risk to the recipient of the xenotransplant, and to those in contact with the recipient and thence the wider public.
The most serious concern is the possibility of transmission of infectious agents including microorganisms from the xenotransplant to the recipient, and the consequent potential for the emergence of a new human infection and possibly disease. A major reason that pigs are considered the donor animal of choice in preference to non-human primates is due to the reduced microbiological burden that they carry. It should be noted that the risk of transmission of microorganisms is not unique to xenotransplantation. Many cases have been documented of transmission of organisms causing disease during allotransplantation procedures.
Cross-species infection (zoonosis) is, however, of particular concern when compared to transmission within a species because the behaviour of an infectious organism in the xenotic host cannot be predicted by its pathogenicity in its natural host. Organisms considered to be benign in their natural host can cause significant morbidity in a zoonotic scenario. Examples include the potentially fatal infections of humans with the Nipah virus of pigs, herpes B virus of primates and hantavirus of rodents (1).
The risk of cross-species infection is also enhanced because the xenotransplant recipient is generally immunosuppressed.
The microorganisms that could be transferred along with the organ, tissue or cell population vary in their potential to establish an infection in the recipient. Viruses such as pig lymphotropic herpesvirus (PLHV), pig cytomegalovirus (PCMV), and pig circovirus (PCV), all of which are highly prevalent in pig populations (see, for example, 2) are able to establish persistent infections and are considered to be potentially oncogenic. Data on activation of cytomegalovirus in pig-to-primate organ transplantation suggests that PCMV may be an important pathogen in immunosuppressed xenograft recipients (3). It has also been reported that PCV type 2 can be transmitted to human cells in vitro (4).
However, the greatest risk of infection may come from those organisms that have an ability to be transferred as an asymptomatic latent entity within the organ. Such organisms include endogenous retroviruses (ERV), and herpesviruses. Pig endogenous retroviruses (PERVs) have been a major source of anxiety and represent possibly the most important safety concern for xenotransplantation as it has been reported that two of the three families of PERV infect human cells in vitro (5).
Unlike other infectious organisms that a pig may carry, PERV viruses are not transmitted between animals as an infectious agent but rather are inherited by all animals as part of their germ-line DNA. Thus, these viruses form part of the genome and are therefore present in every cell. The amount of virus present varies between pig species, but it has been asserted that on average approximately 50 copies of the virus are present in every cell and conventional breeding techniques are not able to remove these viruses from pig populations (6). The presence of infectious organisms, and particularly PERV viruses, in pig cells is thus a potential barrier to the future of xenotransplantation.
It would therefore be desirable to have a method for producing pig cells, tissue and/or organs suitable for xenotransplantation that are significantly reduced in organisms that may be able to be transmitted to a human recipient and thereby significantly reduce the risk of xenozoonotic infection. More specifically, it would be desirable to produce pig cells, tissues and/or organs for xenotransplantation which have a low PERV copy number, thereby minimising the risk of transmission to the xenotransplant recipient. It is an object of the invention to go some way towards achieving these desiderata and/or to provide the public with useful choice.